Recovery of gold

ABSTRACT

Gold is recovered from dilute solution by complexing with malononitrile, followed by sorption on, and elution from, an anion-exchange resin having both weakly basic and strongly basic groups.

m Patent [151 9 Scheiner ml ml. [45] ,MM W, W? 11132 l5 REQUVERW "OFGOLD Wilwmm [72] Inventors: Bernard J. Schelner, Sparks; llkoalld 1E.UNITED STATES PATENTS Llndt R ,b th fN H av 2,648,601 8/1953 Byler et al..75/105 [73] Assignee: The United States oil America as 2,753,2587/1956 Burstall et al ..75/1 18 represented by the Secretary (all thelhriteri- 2,839,389 6/1958 Kember et al. ...75/1 18 or 3,542,540 11/1970Heinen et a1 ..75/101 [22] Flled: Sept 1969 Primary Examiner-L. DewayneRutledge [21] Appl.No.: 862,445 Assistant Examiner-G. T. OzakiAttorney-Ernest S. Cohen and William S. Brown [52] U.S.Cl ...75/ll0ll R,75/101 BE, 75/118 57 AMSTRACT [51] 1m. (31. ..C22lb 111/0 1 [58] Fieldofisearclh ..75/101 R, 101 BE, 118; Gold is recovered from dilutesolution by complexing with 260/430, 429 J malononitrile, followed bysorption on, and elution from, an

anion-exchange resin having both weakly basic and strongly basic groups.

5 Claims, No Drawings RECOVERY OF GOLD Recovery of gold from ores isconventionally accomplished by leaching with cyanide solution. Accordingto this process, crushed gold-bearing ore is contacted with aqueouscyanide solution in the presence of oxygen to form the solubleaurocyanide complex, Au(CN)- The gold is then recovered from theresulting cyanide leach slurry by a variety of processes includingsorption of the complex on strongly basic ion exchange resin (US. Pat.No. 2,753,258) and on an exchange resin containing both strongly basicand weakly basic groups (US. Pat. Nos. 3,001,868 and 3,317,313).However, the aft'rnity of these resins for the Au(CN) complex is veryhigh and elution of the complex from the resin has proven to be a strongdeterrent to practical application of resins for processing the leachslurries, particularly where the slurry is obtained from leaching oflow-grade ores.

It has now been found, according to the process of the invention, thatthe difficulties of the prior art processes can be largely overcome bythe use of malononitrile, rather than cyanide, as complexing agent.Leaching with malononitrile solution is accomplished in essentially thesame manner as with cyanide solution, i.e., the crushed gold-bearing oreis con tacted with aqueous malononitrile solution. The presence ofoxygen is, however, not essential. In addition, effective leaching withmalononitrile requires the use of a basic solu tion, suitably having apH of about to 12. This pH value is conveniently and economicallyachieved by addition of lime to the leach solution; however, other basessuch as sodium hydroxide may also be used for this purpose. Theconcentration of the malononitrile in the solution is usually from about0.05 to 0.25 percent by weight, and the ratio of malononitrile solutionto ore will generally range from about 1.5 to 3. Optimum values of thesevariables will, however, vary considerably depending on the type of andamount of ore, as well as the type of resin used for subsequent recoveryof the complex.

Although preparation of the malononitrile complex is similar to that ofthe cyanide complex, the two are not analogous. The bonding orbitalbetween gold and cyanide in the gold-cyanide complex is SP hybridized,whereas in the malononitrile-gold complex the bonding orbital is SPhybridized. Malononitrile in basic solution forms the malononitrilecarbanion which attacks gold to form a negatively charged species whichhas the probable formula AU[CH(CN)22 Following leaching, the gold, inthe form of the malononitrile complex, is recovered from the leachsolution by sorption on an ion-exchange resin having both weakly basicand strongly basic groups. These ion exchange resins are conventionaland are disclosed in the above-mentioned US. Pat. No. 3,001,868 and3,317,313. lt has now been found, however, that the effectiveness ofthese resins for recovery of gold from leach solutions is much greaterin the case of the malononitrile complex than in the case of the cyanidecomplex. Furthermore, the malononitrile complex is much more readilyeluted from the resin. As a result the use of malononitrile ascomplexing agent results in a process that is practical for recovery ofgold from dilute solutions, even where the solutions are obtained fromleaching of low-grade gold ores or other source materials.

As stated above, the anion-exchange resins employed in the process ofthe invention are conventional and a wide variety of suitable resinswill be apparent to those skilled in the art. However, the preferredresin consists of a poly(styrene-divinylbenzene) matrix havingquaternary ammonium groups as strong-base functional exchange groups andtertiary amine groups as weak-base functional exchange groups. These areconventionally prepared by chloromethylation of the copolymer matrixbeads, followed by swelling of the beads in an organic swelling agentand reaction of the resulting beads with a secondary amine to convertthe chloromethyl groups to tertiary amine groups. The resultingweak-base resin is then reacted with an alkyl halide or sulfate toconvert a portion of the tertiary amine groups to strongly basicquaternary ammonium groups.

The optimum amount of resin employed for sorption of the gold willdepend on a variety of factors such as the specific resin employed,concentration of gold in the original solution, types and amounts ofother metallic ions in the solution, etc., and is best determinedempirically. The resin and the solution of the malononitrile-goldcomplex may be contacted by means of any conventional procedures, suchas addition of the resin, in the form of beads or granules, to thesolution, or by passing the solution through a column of the resin. Foroptimum sorp tion of the gold complex, the pH of the solution should bein the range of about 10 to 12.

Recovery of the gold from the resin is conveniently accomplished byelution. The preferred eluants are the mineral acids, i.e., sulfuricacid, nitric acid or hydrochloric acid. Other eluants such as liquid ionexchangers containing a Pqll group may, however, also be used. When amineral acid is employed, the preferred concentration of acid in thesolution is about 10 percent by weight; however, concentrations of about1 to 10 percent may be used.

The invention and its advantages will be more specifically illustratedby the following example.

EXAMPLE Two-thousand grams of a carbonaceous ore containing 0.25 ouncegold per ton of ore was slurried with 6 liters of water. Five grams ofmalononitrile, 20 g. of lime and 200 ml. (approximately 70 g.) of 20-40mesh granular anion-exchange resin were then added to the slurry. Theresin was the preferred type described above, having both weakly basicand strongly basic amine functional exchange groups and was in thechloride form. The resulting slurry had a pH of about l l .5.

The slurry was stirred for 24 hours and the resin removed by screening.The resin was found to contain percent by weight of the gold in theoriginal ore sample.

The gold-containing resin was then placed in a Al-inch diameter columnand eluted with 10 percent sulfuric acid. Elution of the gold was nearlycomplete with I25 ml. of the acid. Similar results were obtained using10 percent nitric and 10 percent hydrochloric acids.

Experiments were then run using sodium cyanide instead of malononitrileas the complexing agent. The pH of the ore slurry was adjusted to 11 andample oxygen was supplied to the slurry by agitation. These experimentswere otherwise identical to those above using malononitrile. Use ofcyanide as complexing agent resulted in only 63 percent by weightextraction of the gold into the resin and only 5 percent of the gold waseluted from the resin. Thus, the use of malononitrile have distinctlysuperior results both in extraction and elution.

What is claimed is:

It. A method of recovering gold from dilute aqueous solution comprisingcomplexing the gold in solution with malononitrile, sorbing themalononitrile-gold complex on anion-exchange resins having both weaklybasic and strongly basic amine groups as the functional exchange groups,and recovering the gold in acidic aqueous solution from the anionexchange resin by elution with a mineral acid.

2. The method of claim ll in which the solution of malononitrile-goldcomplex is formed by leaching of gold ores with an aqueous solution ofmalononitrile.

3. The method of claim l in which the pH of the aqueous solution ofmalononitrile-gold complex is from about 10 to l2.

4. The method of claim l in which the weakly basic amine groups of theanion-exchange resin are tertiary amine groups and the strongly basicamine groups are quaternary ammoniurn groups.

5. The method of claim it in which the mineral acid is sulfuric acid.

2. The method of claim 1 in which the solution of malononitrile-goldcomplex is formed by leaching of gold ores with an aqueous solution ofmalononitrile.
 3. The method of claim 1 in which the pH of the aqueoussolution of malononitrile-gold complex is from about 10 to
 12. 4. Themethod of claim 1 in which the weakly basic amine groups of theanion-exchange resin are tertiary amine groups and the strongly basicamine groups are quaternary ammonium groups.
 5. The method of claim 1 inwhich the mineral acid is sulfuric acid.